This project will evaluate risks to human respiratory health from interacting air pollutant gases likely to form acid fog. In fog droplets, sulfur dioxide (SO2) and fomaldehyde (HCHO) are likely to form a strong acid--hydroxymethanesulfonic acid (HMSA) or its salts. If such droplets were inhaled into the lungs, HMSA might cause injury by releasing hydrogen ions, and/or by decomposing to release SO2 and HCHO (thus bypassing normal upper respiratory defenses against these gases). The initial experimental effort will develop controlled generation of the pollutants in our human exposure chamber, in combination with existing fog-generation techniques. Pollutant and fog water concentrations will be within or modestly above "worst-case" ambient levels. Bulk fog water collection, filter sampling, and real-time gas and particle monitoring will be employed to characterize the chamber atmosphere. Because ambient acid-forming reactions may not be repoducible in laboratory air, HMSA-contining pollution will be simulated by generating sulfuric acid and the sodium salt of HMSA in fog droplets, along with SO2 gas. Once the chamber atomospheres are well characterized, we will proceed to expose volunteers (beginning with investigators) under conditions simulating typical ambient exposures (one hour duration, intermitten heavy exercise required). Subjects in normal health will be exposed to the fog-pollutant mixture and to its separate components, to identify any toxicologic interaction (i.e. any excess effect of the mixture, as determined by unfavorable changes in respiratory function or symptoms). If an interaction is found, more detailed followup studies will be undertaken to determine which pollutants contribute to the observed interactions, and at what dose levels those interactions occur. If no toxicologic interaction is found in normals, the study will be extended to a potential "high-risk" group--asthmatics. The same basic experimental strategy will continue; however, asthmatics will require more extensive investigations of their reactivity to the separate pollutants.